FIELD OF THE INVENTION
The present invention relates to a radio scanning system using acoustical surface waves (SW radio scanning system), having a transceiver unit and sensor elements that define at least one parameter to be scanned, a transmitter in the transceiver unit transmitting question signals to the sensor elements, and a receiver in the transceiver unit receiving and evaluating answer signals transmitted back by the sensor elements.
Such radio scanning systems are known, for instance, from German Published, Non-Prosecuted Patent Application DE 42 17 049 A1, corresponding to U.S. application Ser. No. 08/270,931, filed Jul. 5, 1994. Such a system includes at least one passive SW sensor for measured-value determination. A measured value is forwarded by radio from a remote measurement location to a scanner, which transmits a question pulse to the sensor element by radio energy. The SW sensor is suitable for contactless measured-value detection. Besides the SW sensor element, an SW reference element is provided for phase discrimination and/or transit time measurement. It can also be learned from the aforementioned patent application that chirped transmission and answer signals can also be used.
If a plurality of sensor elements are spatially dispersed within such a system, then the receiver must be capable of distinguishing, in a central transceiver unit, between the answer signals of the various sensor elements. That can be attained in a manner which is known per se by assigning each sensor element a specific code sequence, that is transmitted by the transmitter in succession in the central transceiver unit, so that after the SW transit time, the autocorrelation peak of the scanned sensor element is transmitted, as the answer signal of that sensor element, to the receiver in the transceiver unit. However, evaluating that correlation peak is made difficult because at the same time cross-correlation signals from all of the other sensor elements arrive in the form of background signals. Since in practice the spatial distribution of the sensor elements is uneven, the cross-correlation signal of a sensor element near the transceiver unit can be stronger than the autocorrelation signal of the more-remote sensor element that is being addressed, and can prevent the evaluation of the latter.